1. Field of the Invention
The present invention relates to a process for preparing branched cyclodextrins.
2. The Prior Art
Cyclodextrins, also termed cycloamyloses, cyclomaltooligosaccharides or Schardinger dextrins, are cyclic (1.fwdarw.4) .alpha.-linked glucopyranoses. They are prepared by the enzymatic degradation of starch. This results mainly in the production of cyclohexaamylose, cycloheptaamylose and cyclooctaamylose, also termed .alpha.-, .beta.- and .gamma.-cyclodextrin.
In addition to these cyclodextrins, branched cyclodextrins are also known. These are also designated branched cyclodextrins, cyclodextrin glycosides or G.sub.n -cycloamyloses, where n indicates the number of .alpha.-D-glucopyranosyl groups in the side chains. These, therefore, are cyclodextrins in which at least one hydroxyl group is linked to glucose, maltose or maltotriose, or generally to substituted or unsubstituted maltooligosaccharides.
It is known to prepare branched cyclodextrins by means of the enzymic glycosylation of cyclodextrin and by means of the pyrolysis of cyclodextrins.
According to Great Britain Patent 2,193,963, the enzymic glycosylation of cyclodextrin is achieved by enzymically reacting a mixture of cyclodextrins and branched maltooligosaccharides using a branch-splitting enzyme.
S. Hizukkuri et al., Biotech. Appl. Biochem. 11, 60-73 (1989), describe the enzymic glycosylation of cyclodextrin achieved by reacting cyclodextrin and maltose in the presence of pullulanase.
U.S. Pat. No. 4,668,626 discloses the simultaneous use of a branch-splitting enzyme and .beta.-amylase for the enzymic reaction.
All enzymic processes for preparing the branched cyclodextrins make use of expensive catalysts, have low conversion rates and necessitate elaborate isolation of the reaction products.
WO 90/01566 discloses the pyrolysis of cyclodextrins at from 135.degree. C. to 220.degree. C.
U.S. Pat. No. 5,118,354 discloses the pyrolysis of cyclodextrins at from 110.degree. C. to 170.degree. C.
Acid-catalyzed pyrolysis results in the production of more unwanted hydrolysis product than of branched cyclodextrins. In addition to this, approximately 50% unreacted cyclodextrin remains behind in the reaction mixture and must be separated off in an elaborate manner. An example of the composition of a reaction mixture obtained in this way is 40.3% .beta.-CD, 22.0% branched CD's and 37.6% acyclic maltooligosaccharides (U.S. Pat. No. 5,118,354, column 6, Table 3, sample).
CA 115: 282425j concerns the acid-catalyzed condensation of cyclodextrins with other sugars. The reaction of .alpha.-cyclodextrin, sorbitol and fumaric acid at 160.degree. C. is disclosed.
The high reaction temperature is disadvantageous in all pyrolysis processes for preparing the branched cyclodextrins. It leads to unwanted byproducts and exacerbates working up and purification of the reaction products. In addition to this, the conversion rates and conversion selectivities are low.
It is likewise known that small quantities of branched cyclodextrins are contained in the mother liquors from the industrial production of .beta.-cyclodextrin (K. Koizumi et al., Carbohydr. Res., 201, 125-134 (1990)).